A dead, drained human body hangs motionless in the web, little more then skin and bones. It's pockets are clearly full and it wears a backpack. Do the PCs dare to try and loot the body?
If they do, they have to make a dexterity check. A failure alerts the giant spider who comes racing out to fight them. A critical failure not only does that, but gets the PC who did it stuck in the web, and his or her life must be saved by his or her fellow PCs. (A possible if nasty way to get rid of that PC who spoils the game for everybody else.) It is up to the GM what the backpack and pockets of the body contains, but it should be worth fighting the spider for. The more dangerous the spider is to the PCs, the better the body should be to loot.
A glowing sword hangs there in the web (in real life, spider web scaled up to thick rope could if the web was large enough catch a jumbo jet in flight) Again, they need to make a dexterity check and succeed to avoid alerting the spider. It is up to the GM how magical the sword is (it could be anything from an ordinary sword that glows to a masterwork magical weapon) but again, it should be worth the risk of fighting the spider for. he more dangerous the spider is to the PCs, the better the sword should be to loot.
A live dog is barking and struggling in the web, and the spider is about to bite it. If the PCs act and kill or drive off the spider and free the dog, the dog will become a friendly, loyal and useful companion. If they do nothing it will become spider food.
4-Male version of Giant Spider
A spider the size of a head hangs in the web looking balefully at the PCs. This is the far smaller male version of the giant spider, and it will pull on the web in such a way as to alert it's far larger mate of the PCs arrival. One good blow with a weapon will kill it, but it's venom is far stronger then that of it's mate.
An egg sac hangs in the web and the giant spider will be far harder to creep past, as it is protecting it's egg sac from pests such as the Dungeon Crab who would happily eat the eggs given the chance. Perhaps it hatches and the PCs are attacked by large numbers of finger sized, venomous, hungry spiderlings.
6-Magical Spell Component
If there is a wizard or witch who is a PC, he or she identifies the web itself as made of an important spell component for x magic spell or potion. As before, sawing free any of the web without alerting it's owner requires a dexterity check, but with a -2 modifier as the target is a piece of the web itself.
A huge glittering gemstone lies in the web, but if the PCs try to loot it, it is in fact a sticky piece of the web itself made as a lure, and will bring the spider out to do battle. Not everything in a web is what it seems and worth the danger of trying to loot it.